Over the past several years, proposals have been made in the United States at the Federal Communications Commission (FCC) and, internationally, at the International Telecommunications Union (ITU) to broadcast radio programs from geosynchronous satellites to receivers in mobile platforms (e.g., automobiles) and in other transportable and fixed environments. Since geosynchronous satellites are located in near-equatorial orbits approximately 42,300 kilometers from the earth's surface, such satellites appear stationary to an observer on the ground. The satellite views roughly one-third of the earth's surface below it, which allows radio broadcast coverage of such a large area or, by using directional antennas on the satellite, a sub-area such as a particular country. This potential national coverage area of many tens of millions of square kilometers for providing radio service throughout the continental United States (or other country/region) is the main feature of satellite radio broadcasting, since normal terrestrial AM/FM radio stations typically cover a much smaller area.
Radio broadcasting from satellites involves use of special receivers in mobile or fixed platforms because of technical implementation and frequency allocation/interference requirements. Consequently, proposals for building such systems have generally used UHF frequencies in the range of about 300 to about 3,000 MHz. FIG. 1 shows a typical satellite radio broadcasting system. Additional satellites can be used with the satellite system shown in FIG. 1 for providing redundancy, additional channels or both. FIG. 1 shows the most important transmission path, the path from the satellite to the mobile or fixed platforms. Since a mobile platform requires an antenna which can receive satellite signals from all azimuths and most elevation angles, the mobile platform antenna gain must be low (e.g. 2-4 dBi gain is typical). For this reason, the satellite must radiate large amounts of radio frequency transmitter power so that the mobile platform receiver can receive an adequate signal level.
In addition to the need for a high power transmitter in the satellite is the need for extra transmitter power, called "transmission margin", to overcome multipath fading and attenuation from foliage. Multipath fading occurs where a signal from a satellite is received over two or more paths by a mobile platform receiver. One path is the direct line-of-sight or desired path. On other paths, the signal from the satellite is first reflected from the ground, buildings, or trucks, and then received by a mobile platform receiver, as FIG. 2 shows. These other paths are interfering in amounts that depend on factors such as losses incurred during reflection.
Among the methods for reducing multipath fading in radio systems, are the following:
1. Providing a second path for a desired signal between a transmitter and a receiver that is physically different from the first path for the signal. This is called space diversity, and is effective where only one of the two paths is strongly affected by multipath fading at any instant;
2. Providing a second transmission frequency for a desired signal between a transmitter and a receiver. This is called frequency diversity, and is effective where only one of the two frequencies is strongly affected by multipath fading at any instant; and
3. Providing signal modulation resistant to multipath fading such as spread spectrum. This method is effective where some resistance results from the large modulated frequency bandwidth used, and some resistance results from the receiver's rejection of an undesired signal's spreading code.
The transmission margin necessary to overcome multipath fading or attenuation from foliage has been both measured and estimated by experts to be in the range of about 9 to about 12 dB for satellite radio broadcast systems operating at UHF frequencies with reception at mobile platforms having roughly 20.degree. or more elevation angle to the satellites. Fortunately, multipath and attenuation from foliage seldom occur simultaneously. However, the need for 9-12 dB transmission margin means that satellite transmitter power must be increased by a factor of 8 to 12 over its initially high level. Radio broadcasting satellites operating at such power levels would be extremely large, complex and costly. To date, no commercial system of this kind is in use because of this high cost.
The systems and methods of this invention overcome these problems, by sending the same radio broadcast signals substantially simultaneously through two or more geosynchronous satellite sources separated by a sufficient number of degrees of orbital arc to minimize the effects of multipath fading and foliage attenuation, as FIG. 3 shows.
A receiver on a mobile or fixed platform receives the two signals through two physically distinct paths in space diversity methods, and selects the stronger signal, or combines the two signals. The signals can be at the same radio frequency using a modulation resistant to multipath interference, or at a different radio frequency, with or without a modulation resistant to multipath. Foliage attenuation is minimized because trees and other foliage are seldom in the line-of-sight to both satellites at the same time.
Receivers on mobile and fixed platforms receive the two signals through the two physically distinct transmission paths and select the stronger signal or combine the two signals to achieve space diversity capability. The signals can be at the same radio frequency and avoid interfering with each other by use of spread spectrum modulation with code division multiple access, or by transmitting the radio signals from each satellite with opposite polarizations (e.g. cross or orthogonal polarizations such as horizontal linear/vertical linear or left circular/right circular). Where isolation of the two signals is achieved by opposite polarizations, any analog or digital signal modulation may be used. Alternatively, the two signals can be transmitted from the two satellites at different radio frequencies, which has the advantage of achieving frequency diversity capability in addition to the space diversity capability. Where different satellite frequencies are used, the signals may be transmitted using any analog or digital modulation.
In preferred embodiments, these systems and methods provide radio broadcasts from geosynchronous satellites with one-eighth or less the power needed with a single satellite. Since satellite cost is directly proportional to satellite transmitting power, the radio broadcast satellite system of this invention uses satellites about one-eighth or less as costly and as heavy as single satellite systems. The reduced satellite mass also permits the use of a lower capability, lower cost launch vehicle. Even if two launch vehicles are needed, the satellite portions of the subject system are still only about one-fourth as costly as a single satellite transmission system.
The subject system substantially improves reception quality by eliminating many blockage outages. Blockage outages occur when physical objects such as buildings or hills lie in the line-of-sight between the satellite and the receiver. As FIG. 4 shows, such blockage seldom occurs simultaneously on both satellite paths. FIG. 4 also shows that signal attenuation from foliage is minimized, because such attenuation results from partial signal blockage.